In precision polishing in process of producing glass optical elements, glass substrates, and semiconductor devices, polishing materials composed of oxides of rare earth elements, mainly composed of cerium oxide and additionally containing lanthanum oxide, neodymium oxide, praseodymium oxide, and/or oxides of other rare earth elements, have been traditionally used. Although other polishing materials, for example, diamond, iron oxide, aluminum oxide, zirconium oxide, and colloidal silica are also known, cerium oxide has been widely used from the viewpoint of the high polishing rate and the surface flatness of polished workpieces. Unfortunately, cerium oxide is unevenly distributed over the world and is not stably supplied. Accordingly, there is a demand for developing a method for producing a polishing material that can polish workpieces with high accuracy at a reduced amount of cerium oxide.
As a method of producing polishing material including highly pure cerium oxide with which precision polishing can be performed in a step of producing optical glass, etc., there is a method of adding salts of carbonic acid, oxalic acid, acetic acid, etc. to an aqueous solution of refined cerium nitrate, cerium chloride, cerium sulfate, etc. to precipitate products such as cerium carbonate, cerium oxalate, and cerium acetate, filtering the precipitate, drying, and then baking to obtain cerium oxide.
For example, Non-Patent Literature 1 proposes a method of heating and mixing an aqueous solution of cerium nitrate aqueous solution, yttrium nitrate aqueous solution and urea to obtain particles with a narrow particle size distribution.
Patent Literature 1 describes a polishing material including a composite grinding material including a core layer formed with a base particle including inorganic material with a smaller relative weight than cerium oxide and a shell layer formed by bonding fine particles including cerium oxide with a particle size smaller than the base particle with a binder on the outside of the base particle. The above describes that such polishing material can be obtained by adding while mixing aluminum oxide sol which is to be a binder to a dispersion liquid dispersed with silicon oxide elementary particles which are base particles, further adding while mixing a dispersion liquid dispersed with cerium oxide particles, separating the obtained solid where the base particle (silicon oxide) and the fine particle (cerium oxide) are bonded through a binder (aluminum oxide), baking the separated solid at 700° C. to 900° C., and grinding the obtained baked product with a dry jet mill. According to this method, a core-shell structure is employed for the particle, the core-shell structure including a base particle (core layer) including silicon oxide and a shell layer including cerium oxide formed by bonding with a binder on the outside of the base particle. Consequently, it is possible to obtain polishing accuracy and polishing rate the same as conventional products while reducing the amount of cerium oxide used.